JP6935475B2 - Cutting material using laser pulse - Google Patents

Description

The present invention relates to a method for cutting a semiconductor material and a laser cutting apparatus.

Fragmentation and scribing are well-known processes in the semiconductor industry, in which cutting equipment is used to process workpieces or substrates such as semiconductor wafers, including but not limited to silicon. Throughout this specification, the term "wafer" is used to cover all of these products. In an individualization process (also referred to as, for example, dicing, shelving, cleaving), the wafer is completely cut, for example, to individualize the wafer into individual dies. In the scribing process (eg, also called grooving, scoring, gorging, farrowing), channels or grooves are cut into the wafer. Other processes may then be applied, such as complete individualization by using physical sewing along the cut channel. Alternatively or additionally, holes can be formed in the wafer using a drilling process. Throughout this specification, the term "cutting" is used to include fragmentation, scribing and drilling.

However, the overall trend of semiconductor technology in miniaturization is to reduce the thickness of the wafer, which reduces the thickness of the wafer, so laser technology is more advantageous for individualization than using mechanical sewing. Is being shown to be. Leveraging high power lasers for such material machining is a significant advantage compared to corresponding mechanical means such as drilling and sewing, and laser machining treats small and fragile workpieces. It has great versatility in doing so.

Laser removal of semiconductor materials occurs due to a rapid temperature rise in a relatively small area where the laser beam is focused, melting local materials, explosively boiling, evaporating and removing them. .. Laser disassembly has challenging requirements, including a delicate balance between process throughput and workpiece (die) quality. Process quality and throughput are determined by polarization such as flux volume, pulse width, repetition rate, polarization distribution, wavefront shape and laser parameters such as phase improvement and wavelength. For example, Patent Document 1 proposes the use of a multi-beam laser cutting technique in which a linear cluster of focused laser beams that can be placed in a linear array of laser spots removes substrate material along a scribing line. And used to cut the substrate radially along the line of removal. The use of multiple beams in this way, as opposed to a single (more powerful) beam, can offer various advantages, especially the advantage of reducing the defect density that occurs during the cutting process.

One of the quantitative assessments of laser process quality is the fracture strength of the die or wafer, which determines the tensile stress when the wafer breaks. Uniaxial fracture tests have typically been adopted to determine the fracture strength of brittle materials and have been adapted to wafer strength measurements. These tests include 3-point and 4-point bending tests, which are typically used to measure fracture strength.

The fracture intensity of the laser-separated wafer is believed to depend on the level of laser-introduced defects that appear after the laser fragmentation process of the wafer, such as microcracks and chip-outs. These defects are caused by high stresses at the interface between the bulk semiconductor material and the local laser machined area. High stresses are caused by acoustic shock waves generated during the process and high temperature gradients between the bulk and the machining area due to chemical changes in the machining sidewalls of the die. The region of the semiconductor material containing such defects is commonly referred to as the "heat-affected region". Fracture strength typically differs between the front and back of the wafer, and in fact there are techniques, processes and wafer layouts that can result in significantly different back and top strengths.

Due to recent developments in ultrashort pulse (USP) lasers, the temporal pulse width of these lasers is shorter than the typical time for electron-phonon relaxation in solids, allowing more delicate wafer processing. Electron-phonon relaxation causes heat transport from the photoexcited electrons to the lattice, and the pulse width is from 1 to less than 10 ps, depending on the specific material being machined. Although USP lasers can provide improved die strength of materials, the productivity of wafer processing systems using such USP lasers often includes, for example, a smaller interaction volume into which thermal diffusion is introduced. Decreases due to the reason.

To increase laser processing speed and thus productivity, the time t ₁ between successive pulses is greater than the time t ₂ between consecutive bursts, i.e. the time between the first pulse p _{1 of the continuous bursts.} A laser pulse configuration has been proposed such that there are two iteration periods or "bursts" of the laser pulse emitted by the laser source so that it is shorter but longer than the laser pulse width Δτ. Bursts are grouped integer of n laser pulses p _n in the period t _2.

Dicing with a burst of ultra-short laser pulses at _{a repetition rate (ie 1 / t 1} ) of several tens of MHz (about 10,000 to about 90,000 kHz) removes material compared to processing using individual laser pulses. It has been shown to be more efficient. However, the heat load caused by the pulses at these repeating frequencies results in a reduction in die strength when compared to the use of a single pulse.

Burst mode configurations have been shown to lead to increased productivity. However, this improvement will reduce the die strength of the fragmented semiconductor dies to some extent that the dies manufactured using this method are unlikely to meet the demands of the current market. Die strength has been shown to be substantially reduced due to the incubation effect, i.e. the cumulative effect of damage and temperature accumulation within the material.

It is an object of the present invention to provide an improved laser cutting method that provides both improved wafer or die strength and improved productivity.

According to the present invention, this object is achieved by performing a burst of a series of laser pulses with a significantly higher pulse repetition frequency.

The use of burst mode with pulse repetition frequencies in the range of 0.1 GHz (100 MHz) to 5000 GHz (5 THz) is relative to the use of burst mode before effective heat transfer from the material region melted by thermal diffusion to the bulk. The efficiency of material removal is further enhanced by the so-called "ablation cooling" effect, in which the hot material evaporates and the temperature of the entire material does not decrease, or at least not substantially increase. Dicing a semiconductor wafer with a burst of laser pulses having a pulse repetition frequency in this range results in an increase in both productivity (ie, higher material removal efficiency) and die strength. This is due to two factors.

i) So that the total energy of the individual pulses in the burst is equal to a particular optimal single pulse energy and the intensity of the laser-guided shock wave generated when the laser pulse enters the target is dramatically suppressed. , The energy of a single pulse within each burst must be reduced. The shock wave is one of the causes of defect formation, and as a result, the die strength is lowered.

ii) Thermal diffusion is suppressed by the ablation cooling mechanism, reducing the size of the area affected by heat, which is one of the significant factors affecting die strength. More specifically, the heat load on the target is reduced by the efficient conversion of the energy of a series of laser pulses into material ablation in the burst, rather than the formation of shock waves and the undesired heat transfer of the material to the bulk. This reduction in heat load reduces the resolidified laser irradiation area, which minimizes the occurrence of structural defects and the formation of cracks in the material.

In addition, such methods allow flexibility in the use of polarization and multiple beam configurations, beam shaping, for example, along with energy tuning (burst shaping) of individual single pulses. Such flexibility allows the process to be optimally tuned for a particular purpose.

Therefore, when burst mode pulse repetition frequencies between 100 MHz and 5 THz are used, the productivity of the laser dicing process and the die intensity of the semiconductor device increase compared to the burst mode pulse repetition frequencies of MHz and kHz.

International Publication No. 1997/029509

According to the first aspect of the present invention, it is a method of cutting a semiconductor material by irradiating the semiconductor material with laser energy.
i) To provide a laser light source adapted to emit a continuous pulse of a laser beam, and
ii) The stage of emitting a laser beam pulse from a laser light source and
iii) In order to irradiate the semiconductor material to be cut, the stage of guiding the emitted laser beam pulse and
iv) Including a step of moving the semiconductor material with respect to a laser beam pulse irradiating the semiconductor material in order to cut the semiconductor material along the cutting line.
A laser light source is adapted to emit a laser beam pulse with a pulse width of 100 picoseconds or less.
In step iii), a method of cutting a semiconductor material is provided in which the semiconductor material is irradiated with a plurality of laser beam pulses at pulse repetition frequencies in the range of 0.1 GHz to 5000 GHz.

Advantageously, multiple laser beam pulses are emitted within bursts of at least two consecutive pulses, each burst containing multiple laser beam pulses, which is not essential for the present invention.

According to the second aspect of the present invention, there is provided a laser cutting device for carrying out the method of the first aspect.

According to a third aspect of the present invention, a laser light source adapted to emit continuous pulses of a laser beam, wherein each laser beam pulse has a pulse width of 100 picoseconds or less.
A laser beam guidance assembly for directing laser beam pulses from a laser source to illuminate the semiconductor material to be cut,
A laser cutting apparatus for cutting a semiconductor material is provided, including a drive assembly for relatively moving the semiconductor material and the laser beam pulse to be irradiated.

Other specific aspects and features of the present invention are described in the appended claims.

The present invention will now be described with reference to the accompanying drawings (not on scale).

The timing schedule of the burst laser pulse is shown schematically. A laser cutting apparatus according to an embodiment of the present invention is schematically shown.

The present invention will be exemplified by reference to FIG. 1, which schematically shows an output laser beam profile containing a plurality of laser beam pulses, such as those which can be emitted by a pulsed laser light source. In FIG. 1, two consecutive bursts, namely burst x and subsequent burst x + 1, are shown in the intensity (y-axis) plot with respect to time (x-axis). For simplification and generalization, intensities are given in arbitrary units (arbitrary units, a.u.).

Each of the plurality of laser beam pulses has a certain pulse energy, and although not shown in FIG. 1, the pulse energy of the first laser beam pulse among the plurality of laser beam pulses is a plurality of laser beam pulses. The laser light source can be arbitrarily controlled so as to have different pulse energies with respect to the second laser beam pulse. As shown, the time t ₁ between successive pulses is shorter than the time t _{2 between the first} pulses p 1 of the continuous burst, but longer than the laser pulse width Δτ, so that the laser beam pulse. There are two consecutive repetition periods or bursts (burst x, burst x + 1). According to the present invention, the laser beam pulse width Δτ is 100 picoseconds or less.

Bursts are grouped integer of n laser pulses p _n in the period t _2. Is the frequency of the laser beam pulses within a burst, pulse repetition rate or burst frequency is given by 1 / t _1. Successive bursts (burst x, burst x + 1) is the frequency of the first laser beam pulse p ₁ of the occurrence of the burst between the frequencies is given by 1 / t _2. The last pulse p _n of the first burst x + 1 is separated _{from the first pulse p 1} of the next burst x + 1 by a time interval t _3. It should be noted that if t ₃ = t ₁ , bursts x, x + 1 are indistinguishable in this figure and appear to be just one uninterrupted burst. Although two bursts x, x + 1 are shown in FIG. 1, the present invention can be applied with any number of bursts, from a single burst to a continuous application of bursts. As shown in FIG. 1, the pulses within each burst have similar intensities and frequencies, but as mentioned above, other embodiments may have different pulse or burst profiles.

According to a specific embodiment of the present invention
_{i) Multiple laser beam pulses in a burst have pulse repetition frequencies 1 / t 1 in} the range of 100 MHz (0.1 GHz) to 5 THz (5000 GHz), optionally 500 MHz (0.5 GHz) to 50 GHz, and less than 100 ps. Has a pulse width of Δτ.

ii) Multiple laser beam pulses are emitted within bursts of at least one pulse, preferably within bursts of at least two consecutive pulses, each burst comprising multiple laser beam pulses.

iii) Each burst may include 2 to 100,000 laser beam pulses (ie, n is in the range of 2 to 100,000), optionally 2 to 1000 laser beam pulses.

_{iv) The inter-burst frequency 1 / t 2} , which is the frequency of the first laser beam pulse in a continuous burst, is in the range of 0.1 kHz to 1000 kHz and optionally in the range of 1 kHz to 100 kHz. ..

v) the burst last laser beam pulses and the first laser beam pulses of the burst subsequent is separated from 10ms by a time interval t ₃ in the range of 1 [mu] s.

A laser cutting device 10 suitable for carrying out the method of the present invention is schematically shown in FIG.

The semiconductor material, the semiconductor wafer 11 in the present invention, is supported on the chuck 13. The chuck 13 and the wafer 11, that is, the wafer 11 are driven at the time of use by the driving unit 14, and there is a relative motion between the wafer 11 and the irradiation laser beam (described later). The ultrashort pulse laser light source 15 is adapted to output the pulse of the polarized laser beam 16 according to the timing schedule as described above. The laser light source 15 can operate to output a laser beam pulse having a pulse width Δτ of 100 picoseconds or less, that is, a pulsed laser beam 16. The pulsed laser beam 16 is guided to the wafer 11 by the assembly. More specifically, the mirror 17 guides the beam 16 and the attenuator / shutter 18 controls the beam. In the present specification, a selectively operable optical polarizing element, which is in the form of a motor-driven half-wave plate 19, is provided so as to be selectively movable for interaction with the pulsed laser beam 16. Be done. Preferably, the wave plate 19 is attached with respect to rotation about the laser beam axis. Therefore, by selectively rotating the wave plate 19, the polarization state of the laser beam 16 can be controlled to be switched. The selective movement is performed by the controlled operation of the motor by a control means (not shown) such as a computer or processor. Another mirror 20 guides the pulsed laser beam 16 to the beam expander 21 to generate a widened beam. The diffractive optical element (DOE) 22 diffracts or divides the widened beam into predetermined patterns of spatially separated output subbeams, which are collimated by the lens 23. Additional mirrors 24, 25 guide the subbeam to the spatial filter 26, which is used to form the desired predetermined beam pattern. The second lens 27 directs the subbeam to another mirror 28, which in turn guides the subbeam to the condenser lens 29. This focuses the laser light on the wafer 11 on the support chuck 13 in a predetermined pattern of irradiation spots. By moving the wafer with respect to the irradiation pulse laser beam 16, the laser beam pulse is applied, thus cutting the wafer 11 along a cutting line (not shown).

It may be advantageous to change the laser beam pulse characteristics between successive bursts depending on the semiconductor material to be cut and the type of cutting operation (grooving, individualization, etc.). For example, with proper control of the laser light source 15, successive bursts can have different pulse repetition frequencies. Alternatively or additionally, for example, a continuous burst by controlling the laser source so that the pulse energy of the pulse in the first burst is different from the pulse energy of the pulse in the second or subsequent burst. The energies transported in may be different.

Alternatively or additionally, the polarization of the emitted laser beam pulses is controlled, for example, by the selective rotation of the wave plate 19 so that the laser beam pulses of different bursts have different laser beam polarization states. May be good. For example, the burst x laser beam pulse may have linearly polarized light parallel or perpendicular to the cutting line, for example, while the burst x + 1 laser beam pulse may have a polarization relative to the burst x laser beam pulse. It may be linearly polarized with polarization directions orthogonal to each other. Laser beam pulses of one or more bursts can also be circularly or elliptically polarized, for example by selectively applying a quarter wave plate (not shown) within the path of the laser beam 16.

As mentioned above, the DOE 22 may be used to diffract the beam 16 into a predetermined pattern of output laser subbeams, which, in combination with the spatial filter 26, may be combined with a desired predetermined pattern of irradiation spots on the semiconductor material. To form. For different bursts, different patterns of irradiation spots are formed, in other words, the laser beam pulses of continuous bursts are such that the pattern of the irradiation spots for the first burst is different from the pattern of the irradiation spots for the next burst. It can be advantageous to be separated. This effect can be achieved in multiple ways, for example by selecting different DOEs for the second burst, or by adjusting the spatial filter 26 between bursts. In an improvement of this technique, the irradiation spots generated in continuous bursts can be spatially separated to irradiate different cutting lines of the semiconductor material. In this way, the first burst can be used to generate the main cut line, while the subsequent bursts are to produce a trench line parallel to the main cut line, but isolated. Can be used.

The above embodiments are merely exemplary, and other possibilities and alternatives within the scope of the present invention will be apparent to those skilled in the art. For example, in the specific embodiment described above, the relative movement between the semiconductor material and the irradiating laser beam pulse is made by moving the semiconductor material while keeping the laser optics stationary, but alternative. In embodiments, relative motion may be provided by moving the laser and / or laser optics while keeping the semiconductor material stationary, or alternatives to the semiconductor material and the laser and / or laser optics. Both systems may be moved.

In the specific embodiments described above, the individual laser beam pulses are generated by using a USP laser. However, it is theoretically possible to use an external beam chopping mechanism that generates individual pulses, for example, using a fast rotating wheel with multiple blocking elements arranged continuously in the laser beam path. Is.

10 Laser cutting device 11 Wafer 13 Chuck 14 Drive unit 15 USP laser light source 16 Laser beam 17, 20, 24, 25, 28 Mirror 18 Attenuator / shutter 19 Motor-driven wave plate 21 Beam expander 22 Diffractive optical element 23 , 27, 29 Lens 26 Spatial filter

Claims (15)

It is a method of cutting a semiconductor material by irradiating the semiconductor material with laser energy.
i) To provide a laser light source adapted to emit a continuous pulse of a laser beam, and
ii) The stage of emitting a laser beam pulse from the laser light source and
iii) In order to irradiate the semiconductor material to be cut, the step of guiding the emitted laser beam pulse and
iv) Including a step of moving the semiconductor material with respect to the irradiating laser beam pulse in order to cut the semiconductor material along a cutting line.
The laser light source is adapted to emit a laser beam pulse with a pulse width of 100 picoseconds or less.
In step iii), the semiconductor material is cut by irradiating the semiconductor material with a plurality of laser beam pulses at a pulse repetition frequency in the range of 0.1 GHz to 5000 GHz.
The plurality of laser beam pulses are emitted within bursts of at least two consecutive pulses, each burst containing multiple laser beam pulses.
The method controls the laser light source so that successive bursts have different pulse repetition frequencies and the emitted laser beam so that the laser beam pulses of different bursts have different laser beam polarization states. A method comprising at least one step of controlling the polarization of a pulse. The method of claim 1, wherein the plurality of laser beam pulses have pulse repetition frequencies in the range of 0.5 GHz to 50 GHz. Each of the plurality of laser beam pulses has a certain pulse energy, and in the above method, the pulse energy of the first laser beam pulse among the plurality is different from the pulse energy of the second laser beam pulse among the plurality. The method according to claim 1 or 2, comprising the step of controlling the laser light source to have. The method of claim 1 , wherein each burst comprises a laser beam pulse between 2 and 100,000. The method of claim 4 , wherein each burst comprises a laser beam pulse between 2 and 1000. The method of claim 4 or 5 , wherein the interburst frequency, which is the frequency of the first laser beam pulse in a continuous burst, is in the range of 0.1 kHz to 1000 kHz. The method of claim 6 , wherein the interburst frequency is in the range of 1 kHz to 100 kHz. The method of any one of claims 4-7, comprising controlling the laser light source such that the transferred energies are different in a continuous burst. Each of the plurality of laser beam pulses has a certain pulse energy, and the method is such that the pulse energy of the pulse in the first burst is different from the pulse energy of the pulse in the second burst. The method of claim 8 , comprising the step of controlling the light source. Step iii) comprises splitting the laser beam pulse into a plurality of spatially spaced sub-beams to generate a pattern of irradiation spots on the semiconductor material, the pattern of irradiation spots with respect to the first burst. However, the method according to any one of claims 1 to 9 , wherein the laser pulses of continuous bursts are divided so as to be different from the pattern of irradiation spots with respect to the next burst. 10. The method of claim 10 , wherein the irradiation spots generated in continuous bursts are spatially separated from each other to irradiate different cutting lines of the semiconductor material. A laser cutting apparatus for carrying out the method according to any one of claims 1 to 11. It is a method of cutting a semiconductor material by irradiating the semiconductor material with laser energy.
i) To provide a laser light source adapted to emit a continuous pulse of a laser beam, and
ii) The stage of emitting a laser beam pulse from the laser light source and
iii) In order to irradiate the semiconductor material to be cut, the step of guiding the emitted laser beam pulse and
iv) Including a step of moving the semiconductor material with respect to the irradiating laser beam pulse in order to cut the semiconductor material along a cutting line.
The laser light source is adapted to emit a laser beam pulse with a pulse width of 100 picoseconds or less.
In step iii), a method of cutting a semiconductor material, wherein the semiconductor material is irradiated with a plurality of laser beam pulses at a pulse repetition frequency in the range of 0.1 GHz to 5000 GHz.
The plurality of laser beam pulses are emitted within bursts of at least two consecutive pulses, each burst containing multiple laser beam pulses.
Step iii) comprises splitting the laser beam pulse into a plurality of spatially spaced sub-beams to generate a pattern of irradiation spots on the semiconductor material, the pattern of irradiation spots with respect to the first burst. A method in which said laser pulses in successive bursts are split so that they differ from the pattern of irradiation spots for the next burst. 13. The method of claim 13, wherein the irradiation spots generated in continuous bursts are spatially separated from each other to irradiate different cutting lines of the semiconductor material. A laser light source adapted to emit continuous pulses of laser beam, where each laser beam pulse has a pulse width of 100 picoseconds or less, and the laser beam pulse is a pulse in the range of 0.1 GHz to 5000 GHz. With a laser source, which has a repetition frequency,
A laser beam guidance assembly for directing the laser beam pulse from the laser light source to irradiate the semiconductor material to be cut.
A laser cutting apparatus for cutting a semiconductor material, comprising the semiconductor material and a drive assembly for relatively moving the irradiated laser beam pulse .
The laser cutting device controls the laser light source so that successive bursts have different pulse repetition frequencies, and the emitted laser beam pulses of different bursts have different laser beam polarization states. A laser cutting device that is capable of performing at least one of controlling the polarization of a laser beam pulse.

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